Perceived esthetics and value of clear aligner therapy systems: A survey among dental school instructors and undergraduate students

ABSTRACT Objective: To investigate the attractiveness, acceptability, visibility and willingness-to-pay for clear aligner therapy (CAT) systems in first-year and final-year dental students and instructors. Methods: A questionnaire designed to collect information regarding esthetic preferences and intentions related to seven CAT systems was handed out to 120 undergraduate students and instructors at the Academic Centre for Dentistry Amsterdam (ACTA). Proportional odds models and population average generalized estimating equation models were used to examine potential association between participant characteristics, esthetic perceptions and CAT systems. Results: Overall, the examined CAT systems received favorable esthetic ratings. Expertise status was significantly associated with willingness-to-pay additionally for CAT, compared to fixed orthodontic appliances. There was no association between sex, previous orthodontic treatment history, satisfaction with own dental appearance and potential interest in treatment and aligner visibility and willingness-to-pay. CAT system was significantly associated with the perceived aligner visibility, acceptability and attractiveness by students and instructors. Conclusions: CAT systems were considered to a great extent attractive and acceptable for future treatment by dental school instructors and students. Willingness-to-pay for CAT systems was significantly associated with expertise status, with instructors appearing more reluctant to pay for CAT.


INTRODUCTION
Public awareness regarding dental appearance has been intensified over the years. Facial and dental attractiveness has been associated with high social competence, intellectual achievement, and favorable psychological development. 1 On the contrary, malocclusion features such as irregular tooth position or inter-arch relationship may negatively affect the perception of overall attractiveness and well-being. 2 Claimed psychosocial effects of dental esthetics may prompt individuals to seek orthodontic care. 3 The rising impact of dental esthetics on social perceptions has raised the demands for adult orthodontics. 4 According to data from the British Orthodontic Society, three quarters of the registered orthodontists have reported an increase of adult private patients. 5 However, orthodontic appliance design and appearance may influence decision to initiate treatment and appliance preference. 6,7 Thirty-three to 62% of adults would decline treatment with visible orthodontic appliances because of poor esthetics. 8,9 To reduce appliance visibility, more esthetically attractive treatment appliances and accessories have emerged, including plastic and ceramic brackets, tooth-coloured wires, lingual brackets and clear aligners.
Clear Aligner Therapy (CAT), originally based on Kesling's tooth positioning device, 10  Inc., San Jose, CA, USA) were introduced as a viable treatment alternative to fixed appliances. Nowadays, more than 27 different CAT products are commercially available, 11 while nearly 9 out of 10 practices in USA routinely perform treatment with clear aligners. 12 Despite the widespread CAT growth, the perceived attractiveness of clear aligners has been rarely investigated. Fixed appliances with colored elastic ties were classified by children as more attractive than clear aligners. 13,14 In contrast, adults rated clear aligners and lingual brackets more favorably compared to ceramic and metallic brackets. 15,16 Moreover, lay adults were willing to pay significantly more for less visible appliances such as lingual appliances and clear aligners for themselves and their children. 15 Study populations in the above-mentioned studies 13-16 comprised laypersons of a broad age range, lacking dental expertise. Given the varying influence of education level and clinical experience on esthetics assessment, this study aimed to investigate the attractiveness, acceptability, visibility and value of CAT systems in dental school instructors and undergraduate students.  In total, seven CAT systems were investigated for the purposes of the study (Table 1).  Jose, CA, US). To ensure the true-life size of the images, the mesiodistal width of the maxillary central incisor was fixed at 8 mm. 16 Figure 1 presents the standardized images acquired by the photographic technique of the study.

SAMPLE RECRUITMENT AND QUESTIONNAIRE DESIGN
Ethical approval for this survey was granted from the Ethics

Committee of the Academish Centrum Tandheelkunde
Amsterdam (ACTA; protocol number, 2018063). All participants Based on previous studies on appliance esthetics, 15,16 a twopart questionnaire was developed. The first part consisted of questions related to demographics (i.e., sex, age, professional expertise), and orthodontic treatment aspects (i.e., orthodontic treatment history, interest in undergoing orthodontic treatment in the future, satisfaction with own dental appearance, and potential willingness to pay more for CAT, compared to conventional metallic brackets).Visibility, attractiveness, and acceptability of the aligners were determined in the second part, using images displayed in random order and coupled with image rating questions. At first, participants were asked to confirm the presence or absence of aligners on standardized smiling images (Fig 2). Germany) reading up to two decimal places. Finally, a yes-or-no question was included to rate aligner acceptability by asking participants if they would be willing to wear the given CAT system in a hypothetical orthodontic treatment (Fig 3).

SAMPLE CHARACTERISTICS
Forty first-year students, 40 sixth-year students and 39 instructors completed the survey. The majority of the participants were females (57.98%), previously orthodontically treated (63.90%), and potentially interested in future treatment (52.90%, Table 2).

WILLINGNESS-TO-PAY
More than 76% of the participants were willing to pay an additional amount to receive CAT instead of conventional fixed orthodontics appliances, mainly up to 500 Euros (Table 3). Fewer instructors intended to pay for clear aligners compared to first-year and last-year students, i.e., 54.05% vs. 85% and 91.18%, respectively. Previously treated participants willing to pay additionally for CAT systems were 2.15 times as many as those not treated.  Students and instructors satisfied with their dental appearance were more eager in paying more for clear aligners, compared to dissatisfied peers, i.e., 77.91% vs. 71.43%, respectively.
Participants interested in future treatment showed a greater willingness-to-pay for CAT than those without interest, and vice versa ( Table 3). In the univariable analysis (Table 4), there was no association between willingness-to-pay and sex, previous orthodontic treatment history, and satisfaction with own dental appearance. Expertise status and interest in future treatment were associated with willingness to pay for clear aligners, but only expertise status remained a strong intention-to-pay predictor in the multivariable analysis (Table 4). In particular, the odds for instructors to pay an additional amount to receive CAT in the future were 72% lower, compared to first-year year students. Figure 4 shows the predicted probabilities for willingness to pay per expertise status, as obtained from the multivariable GEE model.

VISIBILITY
Females, instructors, earlier orthodontically treated or participants interested in future treatment were more capable of identifying CAT systems on the photographs, compared to males, students and those without experience or interest in treatment ( Table 3). The distribution of visibility responses depending on presence of CAT system are tabulated in Table 5.

Willingness-to-pay (probability)
Sixth-year student  was associated with expertise status treatment history, interest in future orthodontic treatment and CAT system. However, in the multivariable analysis, only the CAT system remained a significant predictor (Table 6).

ACCEPTABILITY
The distribution of acceptability responses per CAT system is tabulated in Table 5. Five CAT systems were found acceptable for future treatment by more than 83% of the participants (Table 5). CAT system was significantly associated with acceptability (p<0.001, Table 7).  Dental expertise did not seem to be a significant predictor in rating aligner esthetics by first-year, sixth-year dental students and instructors. Unlike evidence supporting the substantially positive effect of clinical training on the assessment of facial and dental esthetics, 21 longer experience in the dental field did not enable advanced year students or instructors to identify significantly more frequently the aligner images than beginner students. However, the present results are in line with reports on dental esthetics assessment, without significant differences between dentists and dental students. [22][23][24] Visibility and acceptability responses as well as attractiveness VAS scores were not associated with orthodontic treatment history of the participants. Orthodontic patients may develop high valued esthetic awareness due to the increased attention paid during treatment appointments. 25,26 The assumed higher esthetic standards of formerly treated individuals were neither confirmed by competence in recognizing CAT systems on the volunteer's images nor by a tendency to assign higher attractiveness ratings.
Female participants were more skilled in identifying aligner presence and more willing to pay for CAT systems than males, but these sex differences did not reach statistical significance.
Comparable preferences for facial, dental and smile esthetics between the sexes have been reported elsewhere among dental professionals. 21,27,28

STUDY LIMITATIONS
This study have some limitations. In accordance with similar studies [13][14][15][16] , CAT systems were examined in a volunteer with wellaligned teeth, not representing the average orthodontic patient.
If this were not the case, probably tooth misalignment such as rotations and crowding could have compromised the appearance of the aligners and the appliance ratings. Technical parameters like aging and discoloring of the aligners were not considered

RECOMMENDATIONS FOR FUTURE RESEARCH
It would be useful to compare expert and lay groups, such as orthodontists and orthodontic residents, against adolescent and adult orthodontic patients or patients' parents. The participants in this survey, as dental professionals, can be considered more trained in identifying deviation from the esthetic norms, in comparison to laypersons 23 . In addition to this, the strict esthetics standards of dentists may not coincide with patients' perceptions. 28 Finally, as this research focused entirely on subjective perceptions, the combined study of material properties and participants' preferences is necessary to gain more insight into CAT esthetics.